Hydraulic Fluids: The Importance of Viscosity in Hydraulic Power Units Although the last winter was wetter than it was cold, hydraulic machines are still likely to have been at risk from the effects of cold. Fortunately most hydraulic parts are manufactured in either steel or iron, which are not seriously affected by temperature. However, what can be affected is fluid; it can turn into a consistency like ketchup. Another concern in cold weather is the impact that the cold can have on seal material. Very cold and the seal material may become brittle. However, most seal manufacturers take this into consideration and develop their solutions for use in Arctic conditions. Even if the seal is not affected by cold weather, the fluid thickness can still be a challenge. Hydraulic fluids are vulnerable to changes in viscosity based on temperature. This is particularly true if a machine has been sitting in the cold. Thick oil is hard to pump and if the distance for it to travel is far, then it can leave limited energy for work once that has been expended on pumping. The solution to cold weather viscosity problems is to choose appropriate hydraulic fluid. Fluid that is labelled with ’22 weight’ oil may help. However, remember that this setting was registered in 40° not at -10°. So what should the rating be for minus ten? The viscosity index (VI) is a measurement to describe the resistance to a change in viscosity when faced with a change in temperature. The higher the number the better as it will be more resistant to change. Viscosity index is something that should be considered when using hydraulic oil. You want an all-weather operation without much downtime. Not being able to run machines due to weather can cost you much in financial matters, and spending out a little more for suitable hydraulic fluid will save you in the long term.